An electric power steering apparatus (EPS) which assist-controls a steering system of a vehicle by means of a rotational torque of a motor, applies a steering assist torque (an assist torque) to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty of a pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque Th of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle θ, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key (IG) signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist command on the basis of the steering torque Th detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for the EPS by means of a voltage control value Vref obtained by performing compensation or the like to the current command value.
A steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor such as a resolver which is connected to the motor 20.
A controller area network (CAN) 50 to send/receive various information and signals on the vehicle is connected to the control unit 100, and it is also possible to receive the vehicle speed Vs from the CAN 50. Further, a Non-CAN 51 is also possible to connect to the control unit 100, and the Non-CAN 51 sends and receives a communication, analogue/digital signals, electric wave or the like except for the CAN 50.
The control unit 100 mainly comprises an MCU (Micro Controller Unit) (including a CPU (Central Processing Unit) and an MPU (Micro Processor Unit)), and general functions performed by programs within the MCU are, for example, shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 50) are inputted into a current command value calculating section 101 which calculates the current command value Iref. The current command value calculating section 101 calculates the current command value Iref, based on the steering torque Th and the vehicle speed Vs with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref is inputted into a maximum-output limiting section 102 which limits a maximum current in accordance with an overheat protection condition, and the current command value Irefh whose maximum current is limited is inputted into a subtracting section 103. A current deviation ΔI (=Irefh−Im) between the current command value Irefh and a motor current value Im which is fed-back is calculated at the subtracting section 103, and the current deviation ΔI is inputted into a proportional-integral-control (PI-control) section 104 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved at the PI-control section 104, is inputted into a PWM-control section 105, and the motor 20 is PWM-driven through an inverter 106 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 107 and is fed-back to the subtracting section 103. An FET is used as a driving device at the inverter 106, and the inverter 106 is constituted by a bridge circuit of the FET.
In such an electric power steering apparatus, since structure members and peripheral members of the apparatus form a resonant system and generate a vibration, a noisy sound or the like due to the resonance of the resonant systems, it is desired to suppress the vibration and the noisy sound and various counter measures are proposed.
For example, in Japanese Patent No. 5456576 B2 (Patent Document 1), technology, which removes mechanical resonant frequency components of the structure members of the electric power steering apparatus such as a column and a rack or a rigid body portion of a vehicle front structure, is proposed. In the Patent Document 1, the mechanical resonant frequency components are removed by using a band cut filter (a band stop filter (BPF)) having a steep attenuation characteristic or a notch filter, or a combination of those filters and a low pass filter (LPF) of the second order or more.
Further, in Japanese Patent No. 5235536 B2 (Patent Document 2), the noise occurred due to the calculation is removed by using the LPF at an assist control end.